1. Field of the Invention
The present invention relates to a noise-reduced internal gear pump that incorporates an inner rotor having an addendum formed by a smooth curve and a dedendum formed by a hypocyloid.
2. Description of the Background Art
The published Japanese patent application Tokuhyouhei 11-811935 has disclosed an internal gear pump that is intended to reduce the noise, improve the mechanical efficiency, and increase the pump life.
FIG. 11 shows the profile of the gear tooth of the internal gear pump disclosed in the Tokuhyouhei 11-811935. The pump combines an inner rotor having an addendum formed by an epicycloid and a dedendum formed by a hypocycloid (the tooth profile is shown in a dotted line) and an outer rotor having an addendum formed by a hypocycloid and a dedendum formed by an epicycloid (the tooth profile is shown in a solid line). An epicycloidal profile fh1 of the dedendum of the outer rotor is formed by the locus of one point on a first formation circle re1 that is circumscribed on a pitch circle P and rolls without slipping on the circle P from a starting point zO. An epicycloidal profile fh2 of the addendum of the inner rotor is formed by the locus of one point on a second formation circle re2 that is circumscribed on the pitch circle P and rolls without slipping on the circle P from a starting point zO′. A hypocycloidal profile fr1 of the addendum of the outer rotor is formed by the locus of one point on a third formation circle rh1 that is inscribed in the pitch circle P and rolls without slipping on the circle P from the starting point zO. A hypocycloidal profile fr2 of the dedendum of the inner rotor is formed by the locus of one point on a fourth formation circle rh2 that is inscribed in the pitch circle P and rolls without slipping on the circle P from the starting point zO′. The formation circles re1, re2, rh1, and rh2 have a different diameter. A clearance CR between the addendum of the outer rotor and the corresponding dedendum of the inner rotor is equal to the difference in diameter between the third and fourth formation circles rh1 and rh2. A clearance CR′ between the dedendum of the outer rotor and the corresponding addendum of the inner rotor is equal to the difference in diameter between the first and second formation circles re1 and re2. When the amount of eccentricity between the outer and inner rotors is “e,” the clearance between the two rotors at the position where the two rotors interlock with each other most closely is nearly equal to the clearance between the two rotors at the position where the two rotors interlock with each other most loosely.
An internal gear pump is required to have a clearance between the outer rotor and the inner rotor to enable the rotors to rotate smoothly. In the pump disclosed in the Tokuhyouhei 11-811935, the clearance is created by providing the difference in diameter between the first and second formation circles re1 and re2 and between the third and fourth formation circles rh1 and rh2. In this case, when the inner rotor is pressed against the outer rotor at the engaging portion, there exists a minimum clearance between each tooth of the inner rotor and the opposed tooth of the outer rotor. Hereinafter this minimum clearance is referred to as an “interrotor clearance” including the expression in the section “Claims.” The present inventors found that when the pump is operated, the interrotor clearance increases abruptly from zero at the engaging portion, causing the noise.